At a time when Grids are more and more used for applications increasing in size and complexity, we lack of studies and analysis about the usage of such large-scale distributed systems. This paper presents the study of the usage of an experimental grid over a one-year period. This analysis reveals a usage made of peaks and gaps. Moreover, we found that the grid nodes are always powered on. This analysis allowed us to propose an infrastructure for resources reservation that takes into account the energy factor (EARI: Energy-Aware Reservation Infrastructure). We validated this new infrastructure by making a replay of the usage traces of the experimental Grid'5000 platform and we present the resulting gains in terms of energy by comparing different energy policies.